Very high cycle fatigue behavior of riblet structured Alclad 2024 thin sheets

• VHCF tests were performed on flat and riblet structured Alclad 2024 thin sheets.
• Specimens show a sharp transition from early failure to very long life.
• This transition is attributed to the threshold for crack growth in the substrate.
• The riblet structure does not significantly harm the fatigue performance.

Fatigue properties in the very high cycle (VHCF) regime have been studied for Alclad 2024-T351 thin sheets. Furthermore, the influence of a riblet-like surface structure has been evaluated. For this we used a rolling technique to form a riblet structure into the Al clad layer. Fatigue testing was performed at ultrasonic frequencies of 20 kHz, in order to ensure a reasonable runtime of the experiments, with fully reversed axial loading (R = −1). The clad material shows a sharp transition from early failure at some 106 cycles to run-outs at ⩾⩾2 × 109 cycles. The corresponding threshold is higher for the structured samples, due to a modified stress distribution near the interface between Al cladding and Al 2024 substrate as well as due to a shorter crack length of the fatigue cracks formed in the cladding. Fatigue cracks are initiated at the surface of the clad layer and grow easily to the interface (cladding/substrate). The fatigue threshold for this compound material can be attributed to the threshold for further crack growth into the substrate.